feat: manga metadata with status, authors, genres, tags, and search filters (0.15.0)

Adds first-class manga metadata across the stack:

- **Status** (ongoing / completed), **alternative titles**, normalized
  **multi-author** support, **curated genres** (13 seeded), and
  **free-form user tags** (case-insensitive, globally shared). Each is
  modelled as its own table joined to mangas; `mangas.author` is
  backfilled into `authors` + `manga_authors` and dropped.
- New endpoints: `PATCH /v1/mangas/:id` (three-state `description`),
  `POST/DELETE /v1/mangas/:id/tags[/:tag_id]`, `GET /v1/genres`,
  `GET /v1/tags?search=`.
- `GET /v1/mangas` now returns `MangaCard` (with authors + genres
  batched in) and supports `?status=`, `?author_id=`, `?genre_id=`,
  `?tag_id=` filters — AND across facets, with empty-array no-op
  semantics for the unnest primitive.
- `GET /v1/mangas/:id` returns the enriched `MangaDetail` with tags.
- Frontend: reusable `Chip` component; manga detail page renders
  authors as chips linking to `/authors/:id` (Phase 2), a status
  badge, alt titles, genres, and tags with inline add/remove (only
  the attacher sees remove); upload form supports multi-author /
  multi-genre / alt titles / status; search page gets a collapsible
  URL-synced filter panel with keyboard-navigable tag autocomplete.
- 126 backend tests (incl. AND-across-facets primitive, case-insens
  author/tag de-dup, transactional create rollback, PATCH semantics
  for missing / null / set on description); 72 frontend tests +
  svelte-check clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/domain/mod.rs

@@ -1,17 +1,23 @@
 pub mod api_token;
+pub mod author;
 pub mod bookmark;
 pub mod chapter;
+pub mod genre;
 pub mod manga;
 pub mod page;
 pub mod session;
+pub mod tag;
 pub mod user;
 pub mod user_preferences;
 
 pub use api_token::ApiToken;
+pub use author::{Author, AuthorRef, AuthorWithCount};
 pub use bookmark::{Bookmark, BookmarkSummary};
 pub use chapter::Chapter;
-pub use manga::Manga;
+pub use genre::{Genre, GenreRef};
+pub use manga::{Manga, MangaCard, MangaDetail};
 pub use page::Page;
 pub use session::Session;
+pub use tag::{Tag, TagRef};
 pub use user::User;
 pub use user_preferences::UserPreferences;